1. Field of the Invention
This invention relates to a four-wheel steering apparatus of a vehicle which can steer both front wheels and rear wheels, and more particularly to a four-wheel steering apparatus provided with a steering angle extracting mechanism which can easily extract the steering angle of the front wheels and has a high degree of freedom of design and an improved rear-wheel steering mechanism.
2. Related Background Art
In a conventional four-wheel steering apparatus, hydraulic type power steering is provided in respective steering mechanisms for front wheels and rear wheels, and the front-wheel and rear-wheel steering mechanisms are connected together through a rack, a pinion and an intermediate rod or the like. A control mechanism for electronically controlling the steering ratio between the front and rear wheels according to the vehicle speed is provided in the rear-wheel side power steering. Accordingly, the steering angle of the front wheels resulting from the operation of the steering wheel is extracted from the position of the rack and is transmitted to the control mechanism by the intermediate rod through the rack and pinion mechanism, and the control mechanism controls the steering angle to be given to the rear wheels according to the steering angle of the front wheels and the vehicle speed taken into account at the same time. By the effect of such control, the driving state of the vehicle becomes stable both during low speed and during high speed.
However, the above-described conventional four-wheel steering apparatus suffers from the following disadvantages.
The extracting mechanism of the steering angle of the front wheels is constituted by a pinion meshing with a rack provided on the tie rod or the like of the front wheels. In the conventional four-wheel steering apparatus, there is provided a plurality of locations in which the rack and the pinion mesh with each other. However, to extract the steering angle by the rack and the pinion and thereby transmit the steering angle, it is necessary to bring the rack and the pinion into accurate meshing engagement with each other without any back-lash and consideration must be given in terms of space during designing so that the positions of meshing engagement and the position of the intermediate rod have to be optimum relative to the vehicle. This leads to the problem that the mechanism for extracting the steering angle of the front wheels becomes complex or the level of freedom of design of the entire apparatus is low. Further, the rear-wheel steering control mechanism often becomes complex and costly. Thus, it has been difficult to realize a light weight and low cost four-wheel steering apparatus.